The present invention relates to a dry process developing apparatus for use in an image recording system which includes a drum-shaped latent image support member, e.g. an electrophotographic copying machine or a facsimile apparatus. More particularly, the present invention relates to a toner recirculating and agitating device installed in such a developing apparatus.
A prior art dry process developing apparatus of the type described includes a developing roller for forming a magnet brush of a toner thereon and made up of stationary magnets and a developing sleeve rotatable around the magnets, a toner supply roller for supplying the toner to the developing roller, a housing for covering the rollers, and a toner recirculating and agitating device adapted to recirculate the toner to an even distribution within the housing.
Where use is made of a known two-component developer in particular, the toner recirculating and agitating device may include a first toner guide plate formed integrally with a doctor blade which regulates the thickness of a toner layer on the developing sleeve, and a second toner guide plate located below and at a spacing from the first toner guide plate to face the latter. The first and second toner guide plates define a toner recirculation path therebetween. The end of the toner guide plate is spaced from the adjacent wall of the doctor blade to define an inlet of the toner recirculation path through which part of the toner scraped off by the doctor blade is returned to the toner supply roller along the second toner guide plate. A second toner recirculation path is defined by an opening formed through an intermediate portion of the second toner guide plate, a screw conveyor positioned just below the opening and surrounded by a casing, and an opening formed through the bottom of the casing.
In the thus constructed developing apparatus, it is a primary requisite that the toner be fully agitated in order to eliminate any uneven distribution of the toner and, thereby, maintain a constant level of developing ability. This in turn requires the toner removed by the doctor blade to be successively fed towards the second toner guide plate and continuously flow therealong. To agitate the toner more efficiently, it is preferable that the toner on the second toner guide plate be dropped therefrom onto the toner supply roller over as large a height as possible. For this purpose, the recirculation path has to be positioned higher than the scraping end of the doctor blade.
The force tending to recirculate the toner results from the rotation of the developing sleeve and its magnitude depends on the force of the magnets, permeability of the toner, frictional resistance between the toner and the developing sleeve, and angle of friction of the toner itself. Should these parameters be excessively large, a substantial energy would be consumed for the drive of the developing apparatus. This is contradictory to the increasing demand for energy saving while speeding up the fatigue of the toner. Meanwhile, should the toner recirculating force be weak, the flow of the toner would become stopped on the second toner guide plate preventing the toner from being sufficiently agitated.
An attempt has been made to overcome the problem discussed above by shifting the doctor blade to as low a position as possible to incline its wall relative to the vertical plane, so that the resistance to the flow of the toner due to gravity may be suppressed. However, this is not a satisfactory solution to the problem concerned. The low position of the doctor blade requires the recirculation path to be also defined at a low level, which cannot cause the toner to drop over a substantial height onto the toner reservoir. This limits the available toner agitating ability. Also, in the resulting low position of the toner recirculating and agitating device, the screw conveyor and the like are apt to interfere with the toner supply roller.